The present application is the U.S. National Stage filing under 35 U.S.C. xc2xa7371 of International Application No. PCT/FR99/00569, filed Mar. 15, 1999, and claims priority under 35 U.S.C. xc2xa7xc2xa7119 and 365 to French Application No. 98/03366, filed Mar. 19, 1998.
The prior international application was published on Sep. 23, 1999 as Publication No. WO 99/47358 in the French language.
The present invention relates to a thermal print device or thermal printer head.
In general, such a device comprises a ceramic plate which constitutes the base support for all of the functional elements and circuitry that connects a line of heater points (a resistive line) to integrated circuits for controlling these points in response to signals received from an external processor.
The well-known principle on which a thermal print head is based lies in applying on/off control of an electric circuit which has a resistive line portion that heats up under the Joule effect when the circuit is powered and then radiates towards a temperature-sensitive medium that is close to said resistive line portion (practically in contact with the line, ignoring its protective coating) so that the color of the medium at the point of contact changes (e.g. going from white to black).
The resistive line extends across consecutive conductor segments. Every other segment is connected to a common conductor on one side of the line, while each of the intervening segments extends from the other side of the line and has electronic means for switching it on and off, which means are housed in an integrated circuit (known as a xe2x80x9cdriverxe2x80x9d) for controlling a plurality of conductors (e.g. sixty-four conductors). A heating point is thus activated by looping the common conductor and one of the above-mentioned intervening segments via a power supply. Electricity then flows through the portion of the resistive line which extends between two consecutive segments connected to the common conductor and extending across said conductor.
The resistive line is usually situated close to one of the longitudinal edges of the ceramic plate. The common conductor extends between this edge and the line. Furthermore, the connections between the print head and firstly the power supply and secondly the processor for controlling the drivers take place beside the longitudinally-extending edge of the plate which is opposite from its edge adjacent to the resistive line. It is therefore necessary to extend the common conductor as far as said edge, and in present print heads this extension takes place along one of the transversely-extending edges of the plate.
The cross-section of the common conductor and of its extension depends directly on the maximum quantity of electrical current they are to carry, and this can be large (several amps) if all of the heating points are activated. For given thickness of this common conductor, its cross-section depends on its width, and the portion of the common conductor which extends across the plate can be about one centimeter wide. This design is common to all print heads, and in particular to those used for printing travel tickets (over a print width lying in the range 60 mm to 120 mm), and it requires the plate to be longer than the print width (by about 8% to 15%). It will be understood that, by construction, the transverse size of the print head is longer than the length of the resistive line and thus longer than the width of the printing to be performed, and that this extra length can be unacceptable in certain applications. Since the present technological trend is towards miniaturizing equipment as much as possible, and in particular information processing equipment, there exists a need to reduce this size so that the transverse size of the thermal printing mechanism is as close as possible to the width of the medium (strip of paper) on which printing is to be performed.
An object of the present invention is to satisfy this need by providing a print head whose resistive line is of a length that is equal to the longitudinal size of the ceramic plate that forms the support of the head. The means of the invention which enable this object to be satisfied also make it possible to envisage a modular structure for print heads so that by juxtaposing modules in the resistive line direction, it is easy to obtain a range of printers of differing print widths.
To this end, the invention thus provides a thermal print device comprising a plane support of rectangular shape that is fitted on one of its faces, parallel to one of its longitudinally-extending edges, with a resistive line extending over conductor segments disposed perpendicularly thereto and extending alternately to one side and to the other side of the resistive line, one of the series of segments coming from a common conductor which is constituted by a conductive layer covering the support, while the resistive line and the conductor segments across which it extends are formed on an insulating layer that covers the conductive layer.
This particular disposition for the common conductor makes it possible to avoid reserving a special portion of the support and in particular a portion in the vicinity of one of its two transversely-extending ends for the purpose of passing the common conductor. As a result, the resistive line can extend longitudinally from one end of the support to the other, thus making it possible for the printing line to be as wide as the support, which means that thermal printer heads can be made of transverse size that can be kept down to a minimum.
Preferably, the insulating layer leaves an uncovered strip of the conducive layer adjacent to one of the edges of the support, forming the zone in which one of the series of conductor segments is provided and extending between said edge of the support and the resistive line, the conductive layer being cut out to form crennelations in the vicinity of the other longitudinally-extending edge of the support to form areas enabling it to be connected to a power supply, which areas are likewise left uncovered by the insulating layer.
The invention also provides a thermal print head comprising at least two modules, each constituted by a device as described above, said devices being juxtaposed and connected in such a manner that their resistive lines are in alignment with each other.
Other characteristics and advantages of the invention will appear on reading the following description of an embodiment given by way of example.